Kabii bvaiid nokduitg and bttben bagnab



K. E. NORDLING AND R. H. B. BENGTZON.

TWISTED BUILEH TUBE.

APPLICATION FILED APR.29.1919.

Patentedept. 9, 1919.

UNITED STATES PATENT OFFICE.

KAEL EVALD NORDLING ANI) RUBEN BAGNAB. BENEDICTUS IBENGTZON, 0F ML, SWEDEN.

TWISTED BOILER-TUBE.

Specification of Letters Patent.

Application led April 29, 1919. Serial No. 293,568.

To all whom t may concern:

Be it known that we, KARL EvALo Nonn- LING and RUBEN RAGNAR BENEDIo'rUs BanerzoN, citizens of Sweden, residing at ml, in the county of Alfsborg, Sweden, have invented certain new and useful Improvements in Twisted Boiler-Tubes, of which the following is a specification.

Twisted or spirally rolled boiler tubes have of late been used more and more on account of a better` utilization of fuel and the eiective spark arresting attained in many cases. It has however been demonstrated, that the efficiency of the spiral form cannot be maintained as it is practically im possible to remove the gradually increasing deposits of soot in the wave-troughs, that the resistance to the passage of the combustion products is too great, and the tubes have a considerably less stiil'ness than corresponding smooth tubes.

The object of the present invention is to remedy these defects and also to increase the eiiciency of the spiral form of tube.

In the accompanying drawing Figures 1-6 show some forms of the invention. Figs. 13 are longitudinal sections of single threaded, double threaded and triple threaded tubes. Figs. i-G are corresponding crosssections perpendicular to the longitudinal direction of the tubes. For the sake of comparison Fig. 7 shows a cross-section of a common single threaded tube having a uniform wave proiile in the longitudinal section. Figs. 8 and 9 show the cross-sections of double and triple threaded tubes having a similar longitudinal profile, if such tubes existed. All figures show tubes of equal diameter, thickness and depth of thread, and thus they may be directly compared.

Tubes according to the invention are characterized by the following features.

In the longitudinal sections, Figs. lf3, the inwardly directed threads l do not ad join each other directly, but are connected with each other by means oi straight or eventually slightly bent portions 2 of the tube. In the cross-section, Figs. 4-6, these portions 2 are to be found as material located at the circumference or in the prox imity thereof, while the threads l clearly app-ear as wave-like inwardly bent portions Y O the tube material. In each of these bends the inwardly directed curve together with the curves connecting the lirst mentioned curve with the straight portions 2 take up so great a part of thecircumference that the corresponding angle of center 3 is greater than 45". In this Way the inwardly bent portion will obtain such an extension in the cross-section that feasibly not more than three are contained in the same section, and thus t'he number of threads is limited to a maximum of three. The angle 5 formed by the common tangente 4. to the inwardly directed curve and to the two intermediate curves connecting said curve with the outer surface of the pipe is obtuse. The sectional form thus defined implies that the inside diameter D of the tube, the depth H and the length L of the threads as well as the pitch P are dependent of each other within certain limits.

By providing the portions 2 between the threads there is gained, as is clearly shown by comparing Figs. 4-6 with Figs. 74), the condition that the sectional area of the gas current will be increased which results in a reduced resistance to the passage of the combustion gases, that the circumference will be enlarged which results in an increased heating surface and an increased efficiency of the boiler, and that the crosssections of the tubes will have a greater moment of inertia and thus an increased stiffness. By introducing the wave-like bending inward of the threads in the crosssection and the limitation ofthe angle 3 the condition is gained that the line in closing the sectional area of the gas current, 2'. e. the gas profile, will be given a form that even in single threaded tubes and still more in double or triple threaded tubes guarantees the helical movement of the gases. The limitation of the angle 5 excludes U-shaped or similar unsuitable thread profiles in which it is difiicult to avoid a reduction of the thickness of the material in the threads, in which latter impurities from the water in the boiler are easily collected. The simult-.meous presence of the threads l, that are bent wave-like inwardly in the longitudinal section, and of the straight or slightlv bent portions 2 results in a lower limit for the pitch of the threads, by which the resistance of flow in the tubes is further limited andthe twisting of the How of v gases is promoted in such a way that the latter may follow the spiral form without slipping over the wave-crests,'which is the case in tubes with the small pitches hitherto known. This limitationof the pitch makes it also possible to remove deposits of soot in the wave-troughs between the threads by means of blowing or brushing out.

In tubes according to the linvention the disadvantages incident to tubes of the construction hitherto known are substantially removed and the eiicieney of the helical forni is essentially increased. The flue gases are forced by the elicacious rotation intimately to come in contact with the tube walls all around the same during an active exchange of heat with an increased elliciency of the boiler, and the fuel particles crumble away simultaneously by the rubbing against the tube walls. This takes place with a comparatively insignificant resistance to flow. The cross-sections insure necessary rigidity to the tubes. @The great pitch prevents deposits of soot 1n the Wave-troughs between the threads, and makes it possible to clean the tubes thoroughly by blowingorhand-tools. vThus it. `is possible always to maintain the efiiciency of the screw form. Of course, the proleand the depth of lthe threads etc. may be varied' without departing from the characteristic .features of the invention as stated.

Having thus described our invention, what we claim as new and desire to secure by Letters Patent is A screw shaped boiler` tube in which the inwardly directed threads are spaced from one another by unaltered portions of the tube wall, said threads forming a plurality of wave-like recesses in the tube limited to not more than three, each recess having such a shape that its subtended angle is greater than 45 and that the'tangents common to said first mentioned curve and the adjacent curves form an obtus'e angle with each other.

In testimony whereof, we have signed our names to this specification in the presence of two subscribing Witnesses.

KARL EVLD NORDLING. RUBEN RAGNAR BENEDICTUS BENGTZON.

Witnesses: c

KARL PAssoLL, Liss THoREsFoN. 

